1. Check the BIOS to see if you are on stock settings. It's possible you enabled an XMP memory profile, which can cause your CPU to overclock automatically in tandem with memory frequency. Reset BIOS to defaults.
2. Check out the hardware monitoring section in BIOS- how are the temps there? Might be you're getting wrong readings in Windows.
3. Check out auto fan control in BIOS. The CPU HSF fan might be forced to run on low speeds at all times.
4. Yes, check if the HSF is seated properly on the CPU.
Good luck, and let us know of any changes.